Geothermal energy is generally seen as a sustainable source of energy and therefore could be part of the solution for the energy transition. However, there is still a lack of clarity about the conditions under which a geothermal system can be used sustainably. This study investigates the sustainable use of a geothermal system. The geothermal production process in a homogeneous geothermal reservoir with one doublet was simulated using SEAWAT. This model provides the possibility to conduct a sensitivity analysis to examine which parameters play an important role in the geothermal production process. The aim of this analysis was to asses the effect on the well temperature and the thermal recharge, i.e. heat flow from the confining layers towards the reservoir. The tested parameters include four geological uncertainties and two production parameters. From the tested geological uncertainties the thickness of the reservoir has the largest effect on the production profile. Also, it has an impact on the effect of all the other parameters tested, especially on the effect of the confining layers. The confining layers play an important role during the simulation of the production process because they control the thermal recharge, and this enhances the lifetime of the geothermal project. The thinner the reservoir, the larger is the effect of the confining layers and the change in its properties and the more impact the thermal recharge has on the production profile. By examining the effect of the production parameters, the aim was to define a sustainable production design and strategy. It was proven that the production rate has a large impact on the lifetime of a geothermal project, and that an increasing well spacing enhances the sustainable use of a geothermal system. To enhance the sustainable extraction of geothermal energy from a geothermal system the production rate should be kept low. When making a production optimisation two rules of thumb apply to maintain a sustainable production. The first rule says that with a doubling of the reservoir thickness, the production rate can be increased by approximately 50%. The second rule says that with an increase of well spacing of 20%, the production rate can be increased by approximately 50%. Overall, this study emphasizes the positive effect of thermal recharge on the production profile and its enhancement on the sustainable use of a geothermal system. We can conclude that with a sustainable production design and strategy the production from a geothermal system can continue for generations.